Recently, optical discs, optical cards, optical tapes and the like have been proposed and developed as media for recording data optically. Among them, the optical disc has attracted attention as a medium capable of recording/reproducing data with large capacity and high density.
For example, in the case of a phase change optical disc, recording/reproduction of data is performed using a method that is described in the following. A recording layer of an optical disc is irradiated with laser light (this power level is referred to as a recording power level and is represented by Pw) that is focused using an optical head and is stronger than a reproduction power, to cause the temperature of the recording layer to rise and exceed the melting point. After the laser light goes through, a melted portion is cooled rapidly to form a mark in an amorphous state. Alternatively, the recording layer is irradiated with focused laser light that causes the temperature thereof to rise up to the crystallization temperature or more and the melting point or less (this power level is referred to as an erase power level, which is represented by Pe) so that an irradiated portion of the recording layer is changed into a crystallized state.
In this manner, a recording pattern including a mark that is an amorphous region and a space that is a crystalline region is formed on a medium, corresponding to a data signal. Data is reproduced by utilizing a difference in reflectance between the crystalline and amorphous regions.
As described above, in order to form a mark on a medium, laser light needs to be modulated and emitted so that the power level thereof is between the erase power level and the recording power level. A pulse waveform used for this modulation is referred to as a recording pulse. A number of recording methods for forming one mark using a plurality of recording pulses is known. The plurality of recording pulses are referred to as a recording pulse sequence.
At present, CLV (constant linear velocity) recording mainly is used for optical information recording media, such as recording DVD and the like. This is a recording technique in which a linear velocity, a transfer rate and a linear density are substantially constant over the entire surface of a medium. In this case, the rotational speed of a medium varies depending on the recording/reproduction position (i.e., radial position) on the medium.
In contrast to this, a CAV (constant angular velocity) recording technique has been proposed in which the rotational speed and linear density of a medium are substantially constant on the entire surface of the medium. In the CAV recording technique, it is not necessary to control the rotational speed of a spindle motor that rotates a medium. Therefore, the spindle motor and its control circuit advantageously can be produced with low cost. In addition, it is not necessary to suspend recording/reproduction until a predetermined rotational speed is attained after seeking of a recording/reproduction position, whereby the speed of access to a medium can be improved.
On the other hand, in this technique, the linear velocity and the transfer rate vary depending on the recording/reproduction position on a medium. Therefore, conditions for laser light irradiation and heating/cooling vary depending on the recording/reproduction position on a medium.
Various recording techniques are known for improving signal quality when a plurality of different linear velocities are used to record a medium. One of them is a method of forming a mark in which a recording pulse sequence is used in a manner that the ratio of a recording power with respect to an erase power is changed or the width of each recording pulse is changed, depending on the recording linear velocity, as disclosed in, e.g., Patent Publication 1 (JP 2001-118245A (pages 5–7, FIG. 1)). Also, a method of forming a mark using a recording pulse sequence in which the duty ratio of each recording pulse is increased, depending on an increase in the recording linear velocity (i.e., increasing the ratio of a pulse width to a channel clock cycle) is disclosed in, e.g., Patent Publication 2 (JP 2001-222819A (pages 3–5, FIG. 2)). Further, a method of forming one recording mark using a recording pulse composed of one rectangular wave in which recording power or a recording pulse width is changed, depending on a recording linear velocity, is disclosed in, e.g., Patent Publication 3 (JP 2001- 155339A (pages 5–7, FIG. 2)).
However, the above-described conventional recording/reproduction method has a problem in that data cannot be recorded with high signal quality and stability when the range of changing the linear velocity is large. Hereinafter, the problem will be described.
When a recording pulse sequence is used for recording at a high linear velocity and a high transfer rate, a short channel clock cycle is required that is used as a reference for generation of the recording pulse sequence. However, there are certain rising and falling times for laser modulation and emission requirements.
FIG. 16 is a diagram showing waveforms of signals for modulating laser light to record a mark, and a waveform of laser light in a conventional recording/reproduction method. For example, as shown in FIG. 16, when ½ of a cycle Tw91 of a channel clock signal is longer than the sum of a rising time TU1 and a falling time TD1 of laser light, the laser light can be modulated and emitted between each of a recording power level Pw, an erase power level Pe and an inter-pulse power level Pbt.
FIG. 17 is a diagram showing other waveforms of signals for modulating laser light to record a mark, and another waveform of laser light in a conventional recording/reproduction method. As shown in FIG. 17, when ½ of a cycle Tw92 of a channel clock signal is shorter than the sum of a rising time TU2 and a falling time TD2 of laser light, the laser light cannot be modulated between a recording power level Pw and an inter-pulse power level Pbt, so the power level of the laser light varies depending on the pulse width of the emitted light. In other words, since the power level is unstable during modulation, a mark having a desired shape cannot be formed stably.
FIG. 18 is a diagram showing still other waveforms of signals for modulating laser light to record a mark, and still another waveform of laser light in a conventional recording/reproduction method. The method of increasing the duty ratio of each recording pulse, depending on an increase in the linear velocity, has the following problem when the linear velocity is high. Specifically, even when ½ of a cycle Tw93 of a channel clock signal is longer than the sum of the rising time and the falling time of the laser light, if a width between each pulse is shorter than the sum of the rising time and the falling time of laser, the laser light no longer can be modulated between a recording power level Pw and an inter-pulse power level Pbt as shown in FIG. 18.
FIG. 19 is a diagram showing still other waveforms of signals for modulating laser light to record a mark, and still another waveform of laser light in a conventional recording/reproduction method. When a single rectangular wave is used for recording at a low linear velocity and a low transfer rate, the relative velocity between a laser spot and a medium is slow and the width of a recording pulse is long. As a result, a heat accumulation effect is large with respect to a medium, and therefore, mark distortion is likely to occur.
For example, when a mark is formed onto a phase change optical disc, heat accumulated in a front portion of a mark diffuses into a rear portion of the mark while the rear portion of the mark is being recorded. As a result, a larger amount of heat is supplied to the rear portion of the mark than the front portion of the mark on the recording layer. Therefore, as shown in FIG. 19, there occurs a phenomenon in which a mark 702 formed on a track 701 of a phase change optical disc has a rear portion larger than a front portion of the mark 702 so that the shape of the mark 702 is distorted, resulting in a deteriorated quality of a reproduced signal.
Further, the duty ratio of a recording pulse signal is changed along the direction of a time axis with respect to the waveform of emitted light by, typically, delaying a recording pulse signal using a delay line or the like. Therefore, a change along the time axis is discrete. Therefore, in the CAV recording technique, the duty ratio can be changed only discretely, while the linear velocity changes continuously. As a result, recording characteristics vary depending on a recording position in the CAV recording technique.
An object of the present invention is to provide an optical information recording method, an optical information recording device and an optical information recording medium that can record/reproduce data onto/from the same medium over a wide linear velocity range with stability and high signal quality.
[Patent Document 1]
JP 2001-118245A (pages 5–7, FIG. 1)
[Patent Document 2]
JP 2001-222819A (pages 3–5, FIG. 2)
[Patent Document 3]
JP 2001-155339A (pages 5–7, FIG. 2)